1. Field of the Invention
The present invention relates in general to Satellite Positioning System (SATPS) receivers, and in particular to a network assisted pseudolite acquisition for enhanced SATPS navigation.
2. Description of the Related Art
Cellular telephony, including Personal Communication System (PCS) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as internet access, has provided many conveniences to cellular system users.
A current thrust in the cellular and PCS arena is the integration of Global Positioning System (GPS) technology, which is a subset of SATPS technology, into cellular telephone devices and other wireless transceivers. For example, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein, describes a method wherein the basestation (also known as the Mobile Telephone Switching Office (MTSO)) transmits GPS satellite information, including Doppler information, to a remote unit using a cellular data link, and computing pseudoranges to the in-view satellites without receiving or using satellite ephemeris information.
This current interest in integrating GPS with cellular telephony stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 20 feet once an emergency call, such as a xe2x80x9c911xe2x80x9d call (also referred to as Enhanced 911 or xe2x80x9cE911xe2x80x9d) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone""s position. Further, GPS and/or SATPS data can be used by the cellular user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via internet maps or other GPS/SATPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
However, since cellular telephones can travel into areas where SATPS signals cannot be reliably received, augmentations to the SATPS system are being researched to support the E911 and other SATPS/cellular applications. SATPS is increasingly being pressed into service in the cellular telephone/PDA/mobile computer application where a solution is required in areas with substantial blockage, such as inside buildings, in subway stations, and other areas where the system RF link budget is unable to sustain communications with mobile units that travel into hostile signal reception environments such a buildings. Pseudolites are well-known commercially available ground-based transmitters which augment the orbiting SATPS constellation with one or more additional transmitters to improve the availability and quality of a SATPS solution. Current pseudolite applications include local-area augmentation system (LAAS) transmitters for precision approach.
It can be seen, then, that there is a need in the art for a method and apparatus for assisting the cellular network in locating cellular telephones. It can also be seen that there is a need in the art for a method and apparatus for assisting the network in locating cellular telephones in hostile signal environments such as buildings. It can also be seen, then that there is a need in the art for methods and apparatuses for assisting a mobile user in a harsh signal environment using extra-network devices such as pseudolites.
To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses methods and apparatuses for assisting the cellular network, or other type of network, with SPS acquisition, tracking, and navigation using extra-network devices such as pseudolites.
The invention comprises a set of pseudolites deployed throughout an area of interest in which the SATPS constellation visibility may be limited, such as in and around some high-rise buildings in an urban setting, and a database of these installed pseudolite locations with their PRN numbers indexed by cell-site. When the cellular telephone is in communication with a particular tower or transceiver, the PRN numbers of all local psuedolites are transmitted to the cellular telephone. The data transmitted for each pseudolite could be simply the pseudolite ID, the ID and position of the pseudolite, or the ID and the complete data message for the pseudolite (which would include it""s position). The phone can then augment the phone""s normal acquisition list with the psuedolites in the area and track the pseudolites to augment navigation capabilities in a harsh environment. The advantage of having the pseudolite ID""s transmitted to the SATPS receiver for addition to the acquisition list is that they are usually not looked for at all, because they are infrequently available. In GPS, for example, pseudorandom code numbers (PRNs) 1 to 32 are reserved for space vehicles, and PRNs 33 through 37 are reserved for pseudolites. Since pseudolites are local area transmitters, these PRNs may be reused in different geographic areas without interference. A conventional GPS receiver, however, would not search ID""s 33 through 37 for potential pseudolites because of their deployment is very rare. Searching for transmitters that have not been deployed wastes power and time in the GPS receiver. The present invention extends the conventional SATPS system by providing the list of available pseudolite PRNs to SATPS systems which may benefit from tracking them because of the obstructed signal environment.
The position of the pseudolite may be transmitted by the cell base station as well. If the position is transmitted, it can be used by the SATPS to compute a prepositioning range and range-rate for the pseudolite for faster acquisition of the pseudolite signal. Alternatively, if the cellular telephone has a position solution which has been transmitted to the cell tower, prepositioning information for a code search can be transmitted to the phone. If the pseudolite position and any required clock correction terms are transmitted by the cell base station to the SATPS receiver, then the pseudolite pseudorange may be used in the SATPS navigation computation before the pseudolite data message has been decoded directly from the pseudolite transmission, thus making it available for use in assisting navigation sooner. If all of the data in the pseudolite data message is transmitted to the SATPS by the cell base station, the data message in the actual pseudolite signal can be stripped off of the incoming signal. In the case of a GPS pseudolite, this allows coherent tracking of the pseudolite signal beyond the 20 millisecond (ms) boundary normally imposed by the data rate in the modulating bi-phase shift keyed (BPSK) code.
In another embodiment of the invention, the pseudolite information is transmitted to the mobile user via a broadcast mechanism, with no communication from the mobile user back to the source transmitting the local pseudolite information. An FM broadcast station sub-carrier would be well suited to this task, because the limited range of the FM broadcast availability may be consistent with the range of the signals transmitted by the pseudolites identified in the FM sub-carrier broadcast. So, for example, in a city with a local FM station, the sub-carrier broadcasts the ID numbers of the pseudolites available in the city. As in the case of the cellular network based system, the broadcast may also include the pseudolite positions, the pseudolite clock corrections, or the entire pseudolite data message. In any of these cases, the SATPS receiver, augmented with the sub-carrier receiver, picks up the ID""s and any other pseudolite information, and is therefore able to acquire, track, and navigate with signals from the local pseudolites as well as the satellites. The resultant navigation fixes may be for use locally by the mobile user, or they could be transmitted to through another network, such as the cellular telephone network, to support E911, fleet management, and other applications. The mechanism for transmitting the pseudolite aiding information can be one-way and need not be associated with the transmission of navigation data from the mobile user.
It is an object of the present invention to provide a method and apparatus for assisting the cellular network in locating cellular telephones. It is another object of the present invention to provide a method and apparatus for assisting the network in locating cellular telephones in hostile signal environments such as buildings. It is a further object of the present invention to provide methods and apparatuses for assisting the network using extra-network devices such as pseudolites.